The present invention relates to a gas sensor installed in an exhaust passage of an internal combustion engine to measure the concentration of a specific gas.
An automotive vehicle engine is equipped with a gas sensor to measure an oxygen concentration or a NOx concentration in an exhaust gas emitted from this engine. The detected gas concentration is used to control the combustion of the engine.
For example, the gas sensor has a gas sensing element extending in a longitudinal direction, a cylindrical insulator having a through hole into which the gas sensing element is airtightly inserted, and a cylindrical housing airtightly supporting the insulator via an annular metallic packing.
A measured gas cover, attached to a distal end side of the housing, defines a space into which the measured gas is introduced. A distal end of the gas sensing element is exposed to a measured gas atmosphere provided inside this cover. An air side cover, attached to a proximal end side of the housing, defines a space into which the air is introduced to provide an air atmosphere.
The clearance between the gas sensing element and the insulator is airtightly sealed. Similarly, the clearance between the insulator and the housing is airtightly sealed.
The gas sensing element has a measured gas side electrode exposed to the gas stored in the measured gas atmosphere and a reference electrode exposed to the air. An ion current or an electric potential difference occurring between the measured gas side electrode and the reference electrode represents the concentration of a specific gas to be measured in the exhaust gas.
According to this arrangement, to assure accurate detection of the specific gas, it is important to provide a reliable sealing for completely separating the measured gas atmosphere from the air atmosphere.
One of the key portions to be surely sealed in the gas sensor is the interface between the insulator and the housing.
FIG. 16 shows a sealing arrangement between the insulator and the housing of a conventional gas sensor. According to this arrangement, an annular metallic packing 1011 interposed between a tapered surface 1033 of an insulator 1003 and a receiving surface 1103 of the housing 1010 determines the sealing property between the insulator 1003 and the housing 1010.
The metallic packing 1011 is brought into face-to-face contact with the receiving surface 1103 and with the tapered surface 1033. A lower surface 1112 of the metallic packing 1011 entirely contacts with the receiving surface 1103. An upper surface 1111 of the metallic packing 1011 entirely contacts with the tapered surface 1033.
However, the tapered surface 1033 of the insulator 1003 has a significant undulation which will give adverse influence to the sealing property between the tapered surface 1033 and the metallic packing 1011.
To compensate this drawback, an additional sealing member such as a power sealing material was conventionally used.